Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Various radiation compositions and imageable elements are described in U.S. Pat. Nos. 6,309,792 (Hauck et al.), 6,893,797 (Munnelly et al.), 6,787,281 (Tao et al.), and 6,899,994 (Huang et al.), U.S. Patent Application Publication 2003/0118939 (West et al.), and EP 1,079,276A1 (Lifka et al.) and EP 1,449,650A1 (Goto).
Other negative-working imageable elements are described, for example, in U.S. Pat. Nos. 6,916,595 (Fujimaki et al.), 6,702,437 (Fujimaki et al.), and 6,727,044 (Fujimaki et al.), Japanese Kokai 2000-187322 (Mitsubishi Chemical Co.), and U.S. Patent Application Publications 2004/0131972 (Fujimaki et al.), 2005/0031986 (Kakino et al.), 2006/0068328 (Aimura et al), and 2006/0199097 (Oda et al.).
Negative-working imageable elements have also been designed with overcoat layers that provide lower barriers to oxygen permeation as described, for example in EP 1,296,187A2 (Oshima). Such overcoat layers apparently include polymers that are more permeable to oxygen than poly(vinyl alcohol).